Methanococcus vannielii formate dehydrogenase exists as (1) a 100,000 dalton form that contains approximately one gram atom of molybdenum and ten each of iron and acid labile sulfur and (2) a larger molecular weight complex consisting of multiples of (1) plus selenoprotein subunit(s). When the organism is cultured in media containing tungsten, in addition to Fe, Mo and Se, only the complex form of the enzyme is produced and Mo is partially replaced by W. The tungsto-enzyme appears to be a more stable form. Both the 100,000 dalton enzyme and the larger Se-enzyme complex reduce a deazaflavin-linked enzyme which in turn reduces NADP ion.